From: Will Deacon <will.deacon@arm.com>
Date: Mon, 5 Feb 2018 15:34:21 +0000
Subject: arm64: uaccess: Prevent speculative use of the current addr_limit
Git-commit: c2f0ad4fc089cff81cef6a13d04b399980ecbfcc
Patch-mainline: v4.16-rc1
References: bsc#1068032

A mispredicted conditional call to set_fs could result in the wrong
addr_limit being forwarded under speculation to a subsequent access_ok
check, potentially forming part of a spectre-v1 attack using uaccess
routines.

This patch prevents this forwarding from taking place, but putting heavy
barriers in set_fs after writing the addr_limit.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>
---
 arch/arm64/include/asm/uaccess.h |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -43,6 +43,13 @@ static inline void set_fs(mm_segment_t f
 	current_thread_info()->addr_limit = fs;
 
 	/*
+	 * Prevent a mispredicted conditional call to set_fs from forwarding
+	 * the wrong address limit to access_ok under speculation.
+	 */
+	dsb(nsh);
+	isb();
+
+	/*
 	 * Enable/disable UAO so that copy_to_user() etc can access
 	 * kernel memory with the unprivileged instructions.
 	 */